The present invention relates to a method for making a thermoplastic or elastomeric product which will cross-link in the presence of moisture on the basis of silane or silane compounds which have been grafted onto the thermoplastic or elastomeric macramolecules.
The German printed patent application 1,963,571 discloses a method for cross-linking polyolefine e.g. polyethylene or a mixed polymer in the presence of a compound which is capable of producing free radicals in the olefin macromolecules. A silane compound of the formula R R.degree. Si Y.sub.2 is grafted onto the polyethylene macromolecules, whereby R is a mono-valued olefinically unsaturated hydrocarbon or hydrocarbon oxi-group; Y is an alcoxi-,acyloxi-,oxime- or a substituted amino group; and R.degree. is either a group R or a group Y or methylene. The silane is grafted upon the macromolecules of the olefine in an extruder and at a temperature in excess of 140.degree. C. The extruder kneads and masticates the material and mixes its components. After the material has resided in the extruder for a certain period of time, the grafted polymer is extruded, granulated and stored. The grafted polymer powder may be mixed later with a catalyst batch, extruded as to the final shape, and the resulting product is then placed into hot water to obtain cross-linking.
These known methods are characterized by a relatively large number of individual steps, such as mixing, grafting, granulating, etc. Moreover, it was found that the material may to some extent cross-link prematurely, i.e. prior to obtaining its final shape. This is particularly so, for example, during cooling of the granulate. Thus, the final product is not always satisfactory. For example, the material may be used to jacket an electrical cable, and the jacket requires particular mechanical as well as electrical properties. As to the latter aspect, it is particularly disadvantageous that the known method results in pockets in the material after having been shaped. The insulative properties are, therefore, locally quite weak, so that, for example, the voltage that can be transmitted is limited. As regards the mechanical properties, the formation of gas pockets may render the surface quite rough, and the jacket, for example, can wear rather rapidly.